In full-duplex hands-free communication system nowadays always Acoustic Echo Cancellers (AECs) are used. A crucial component of such an AEC is an adaptive filter, often referred to as an echo filter that models a physical path between a loudspeaker and a microphone and tries to predict an echo on the microphone caused by the loudspeaker. A signal of a far-end talker on the opposite side is amplified and fed to the loudspeaker. The echo is generated and picked up by the microphone together with the speech signal of a near-end talker, if present. The adaptive filter makes a replica of the echo signal, which can be subtracted from the microphone signal. This is done by estimating (a part of) the impulse response between the loudspeaker and the microphone.
The adaptive filter, typically implemented as an N-point transversal filter, updates its coefficients by correlating the residual signal, which is a difference between the microphone signal and the replica of the echo signal, with the input data of the transversal filter. However, this works well if only the echo is present. If also the near-end talker signal is present, then problems can occur, since the adaptive filter will diverge due to the presence of the near-end talker signal. If the far-end talker signal and the near-end talker signal are correlated these problems are more severe. A solution to this problem is to reduce the amount of adaptation in case of double talk and in case of severe double talk even to stop the adaptation. A well-known solution is to use step-size control like e.g. given in A. Mader, H. Puder and G. U. Schmidt, “Step-size control for acoustic echo cancellation filters—an overview”, Signal processing 80 (2000) pp. 1697-1719.
The updates of coefficients of the adaptive filter are then dependent on the acoustic coupling such that the step-size of the adaptive filter decreases once a near-end speaker becomes active. The acoustic coupling is then estimated as the ratio of powers of the far-end talker signal to the microphone signal. However, for double talk situations this acoustic coupling is estimated too large, such that the decrease in the step-size of the adaptive filter becomes too small and the adaptive filter diverges from its optimum solution.